1. Field of the Invention
The present invention relates to a structure of a bonding apparatus and to a bonding method using the bonding apparatus.
2. Description of the Related Art
In the case of ultrasonic or thermo-compressive bonding of thin metallic wires to pads on electrode sections in a semiconductor chip and electrodes on a circuit board using a wire bonding apparatus for connection therebetween, the surface state of the pads or electrodes is critical for the bonding quality. That is, if the metal-layer surfaces of the pads or electrodes are contaminated, or water or foreign material adheres thereto, there occur problems of poor electrical bonding between the pads or electrodes and the wires as well as of reduced mechanical bonding strength. It is, therefore, often the case that a surface treatment is performed before the bonding process to remove contamination, water, or foreign material on the pads or electrodes.
Such a surface treatment for removal of contamination, water, or foreign material on metal surfaces has conventionally been a wet cleaning process in which the metal surfaces are sprayed with a water removing solvent and an organic contamination removing solvent and then dried and electrically neutralized under an inert gas atmosphere. However, apparatuses for such wet cleaning are required to supply, discharge, and dispose cleaning liquid, suffering from a problem in that the overall apparatus size increases to be less easily incorporated into a wire bonding apparatus.
Hence, there has been proposed a dry cleaning method in which metal surfaces are irradiated with plasma to be cleaned without using any solvents. Japanese Unexamined Patent Application Publication No. 2006-332152, for example, proposes a metal surface cleaning method in which the surfaces of pads on a semiconductor chip are irradiated with argon gas plasma. Japanese Unexamined Patent Application Publication No. 2006-332152 also proposes a method for good bonding in which the spark voltage and/or current when forming wires into balls are regulated to increase the crystal grain size and the balls are softened to thereafter be pressed and bonded to the pads on the semiconductor chip. This method aims at good bonding by softening the balls to increase the deformation of the balls when pressed against the pads, so that shells which is formed by oxide films and/or substances that adheres on the surfaces of the balls when the balls are formed, are broken through the deformation to expose newly emerging metal surfaces, and pressing the newly emerging surfaces against cleaned pad surfaces.
Japanese Unexamined Patent Application Publication No. 2006-332151 proposes a method in which when a semiconductor chip is flip-chip mounted on a lead frame or a substrate, electrodes on the surface of the lead frame or substrate are irradiated with argon gas plasma to be cleaned, while the surfaces of stud bumps formed on electrodes on the semiconductor chip are exposed to laser radiation so that the stud bumps have an increased crystal grain size to be softened, and then the stud bumps are pressed against the electrodes. This method aims at good bonding by increasing the deformation of the stud bumps when pressed against the electrodes on the lead frame or substrate, so that shells which is formed by oxide films and/or substances that adheres on the surfaces of the stud bumps are broken through the deformation to expose newly emerging metal surfaces, and pressing the newly emerging surfaces against cleaned pad surfaces.
In addition, Japanese Unexamined Patent Application Publication No. 2001-68500 proposes a method of bonding wires to pads with lower load without using ultrasonic energy in which a wire extending from a tip end of a capillary is formed into a ball through a micro-arc process and the ball in a melt state is bonded directly to a pad, and also a method in which the metal surfaces of electrodes are irradiated with argon micro-plasma arc to be cleaned, and then wires are bonded to the electrodes.
The related arts disclosed in Japanese Unexamined Patent Application Publication Nos. 2006-332152 and 2006-332151 are directed to a method that aims at good bonding by softening the balls or stud bumps so that shells which is formed by oxide films and/or substances that adheres on the surfaces of the balls or stud bumps are broken when pressed against the electrode surfaces to allow newly emerging metal surfaces to come into contact with the electrode surfaces, and not to removal oxide films or substances that adheres on the surface thereof. Therefore, in the related arts disclosed in Japanese Unexamined Patent Application Publication Nos. 2006-332152 and 2006-332151, the oxide films and/or substances that adheres on the surfaces of the balls or stud bumps may be placed between the metal surfaces and the balls or stud bumps during bonding, resulting in poor bonding.
Also, in the related arts disclosed in Japanese Unexamined Patent Application Publication Nos. 2006-332152, 2006-332151, and 2001-68500, the surfaces of the pads on the semiconductor chip or the electrodes on the lead frame or substrate can be cleaned by irradiation of plasma thereto, but balls or wires to be bonded to the pads or electrodes cannot be cleaned, whereby substances that adheres on the surfaces of the balls or wires may cause poor bonding.